:rotating_light: [`FA4`] Initial support (#42435)
* initial implementation
* CB support
* change how we call item on max_seq_len_q/k
* fix
* tests
* fix fa2 clash
* unify the fa dispatch
* fix
* modernbert...
* oops
* parity test
* style
* nit
* fixup imports for fa4
* enable attention sinks, fixup logits checks in parity test
* style
* change dispatch logic and introduce lower bound for FA
* style
* fix test
* min fa2, avoid 2x device sync
* style
* simple min version instead of list
* fixup error message on non init check
* fixup up non init check a tad more
* refactor some FA constants out to main fa utils
* new marker for all fas needed
* oops
* style and make the fa kernel fallback generalized
* default none...
* more refactors
* style
* fix
* this test faulty even on main, xformers can handle any shape apparently yikes
* lets make this more robust, we should check for none within...
* fix
* oops
